The present invention relates generally to telecommunication test equipment used to simulate telephone networks in a laboratory setting and more particularly to a device for providing an interface to enable two central office simulators to be used concurrently in a series or cascade topology to perform testing procedures.
Traditional test equipment provides an analog subscriber line interface for connection to a handset, modem, fax machine, data terminal, or other communication device capable of utilizing analog phone lines for transmission. In an analog telephone test system, each telephone or other communication device ("analog subscriber unit") is typically connected by a pair of wires ("tip" and "ring" wires or, cooperatively, "subscriber lines," "subscriber loop" or "phone lines") to the testing equipment in a manner similar to the subscriber connection in a standard telephone network. The test equipment provides the required functionality such as on-hook/off-hook detection, DTMP or pulse dial signal interpretation, supervisory signal generation (e.g., ring, ringback, busy, and dial tone), etc., and other control functions that may be necessary for call initiation, progression, and termination. The test equipment also provides simulation of various signal impairments and characteristics of a telephone transmission system and/or a telephone company central office ("central office" or "CO") and/or a CO-to-CO communication link.
Test equipment devices for use in telephone transmission system simulation may provide an end-to-end link for connecting two analog subscriber units, thereby enabling the two analog subscriber units to establish a communication link. One or both of the subscriber units may be units undergoing testing, or alternatively, one may be a reference subscriber unit. Typically, test equipment devices that simulate network impairments have the capability to mimic analog and/or digital sources of distortion commonly found in telephone network environments such as: signal echoes, additive noise, gain distortion, delay distortion, frequency shift, phase jitter, PCM coding distortion, PCM bit robbing, and the like.
One impairment simulator device presently available is the TAS 100 Series Telephone Network Emulator 10 (hereinafter referred to as a "TAS") depicted in FIG. 1, available from Telecom Analysis Systems, Inc., of Eatontown N.J. The TAS 100 Series test units have impairment generators 12 and 14 and echo generators 16 and 18 placed between two central office emulators 20 and 22 for providing connectivity between two analog subscriber units 24 and 26 over analog subscriber lines 28 and 30. The TAS also provides a digital interface bus (GPIB or RS-232) (not shown) for connection to a computer for remote control operation. The actual control is implemented through a Gemini test unit which receives commands from a personal computer and conveys the necessary control information to the TAS unit. The computer can be used to automatically configure the TAS in a number of different ways to emulate various line conditions. For a complete description of the features associated with the TAS 100 series products, see the 100 Series Telephone Network Emulator Operations Manual, Part Number 2700-3015, Version 2.11 (1995), which is hereby incorporated by reference.
Because most modem telephone systems are digital, and route digital information from CO to CO, there are test units that provide a digital interface, such as a T1 connection, on one side of the simulated communication link and a standard analog interface on the other side of the simulated communication link for connection to an analog subscriber unit. One such device is an Ameritec AM7 C.O. Simulator (hereinafter referred to as an "AM7"), available from Ameritec Corporation, located in Covina, Calif. The AM7 can serve as a central office, toll office, equal access switch, tandem switch, etc. The AM7 can provide 1.544 Megabit PCM and 2.048 Megabit PCM port cards, and each 1.544 Megabit port card provides access to twenty four channels associated with a T1 "span." The AM7 also provides 2-wire analog loop start or ground start interfaces. For a complete description of the features associated with the AM7, see the Ameritec AM7 C.O. Simulator User's Manual, part number 18-0014 (Mar. 6, 1991), which is incorporated herein by reference.
Subscriber units may also be digital in nature and have a digital interface to the phone system. Although digital subscriber units may ultimately provide an analog output such as audio or video signals, their interconnection to the communication network is generally of a digital nature by way of a DS0, DS1, T1, E1, ISDN, etc. (hereafter, the term "subscriber unit" is used to refer collectively to analog and digital subscriber units). Typical digital subscriber units include ISDN modems, devices interconnected to a Digital Data System (by way of, e.g., a Digital Data Port) and hub servers having direct digital access to the telephone network of the type disclosed in U.S. Pat. No. 5,528,595 to Walsh, et al., entitled "Modem Input/Output Signal Processing Techniques," and U.S. Pat. No. 5,577,105 to Baum et al., entitled "Telephone Call Switching and Routing Techniques for Data Communication." The contents of the above-named patents are incorporated herein by reference.
Simulated communication links suitable for testing of digital subscriber units may also be configured using test equipment of the type described above. Specifically, the AM7 provides a T1 interface that may be configured for connection to a digital subscriber unit. The AM7 also provides a central-office type analog subscriber line interface for connection of an analog subscriber unit. Thus, the AM7 can provide a simulated communication link between an analog subscriber unit and a digital subscriber unit. A block diagram of a simulated communication link using an AM7 50 is shown in FIG. 2. A digital subscriber unit 52 is connected to an AM7 50 by way of T1 line 54. The AM7 50 provides an analog loop circuit 56 for connecting an analog subscriber unit 58.
In certain circumstances it may be desirable to simulate a communication link between a digital subscriber unit and an analog subscriber unit. In such a situation it is desirable to cascade the two types of test equipment described above. Specifically, it may be desirable to have a T1 interface by way of an AM7 to an analog line impairment system by way of a TAS that also provides connection to an analog subscriber unit. This would allow a unit designed to have T1 access to a telephone network to be tested with a remote analog subscriber unit. Neither the TAS device nor the AM7 alone is satisfies all of the testing and interface requirements.
In a manner similar to a standard telephone network, the test equipment described above provides DC power, or "talk battery" power, on the analog lines (tip and ring lines) to enable operation of circuitry within the subscriber units connected to those lines. As part of the central office simulation functionality the test equipment also provides a ring voltage signal to the analog subscriber unit to indicate to the subscriber unit that a call is incoming. Typically, the talking battery is approximately 48 volts across the tip and ring when the subscriber unit is on-hook. The analog subscriber unit provides an open circuit (or a high impedance, such as 10 K ohms, when in the on-hook condition. In a "loop start" circuit, the analog subscriber unit generates an off-hook condition by providing a termination of approximately 600-900 ohms, i.e., by closing, or "looping" the tip and ring to form a complete electrical circuit.
One disadvantage associated with the above described test equipment is that neither provides a means of serially connecting the test equipment devices. That is, the only interfaces available for interconnection are analog subscriber-type interfaces similar to those provided by a central office. The analog interfaces 56, 28, and 30 of both pieces of test equipment are designed to provide the loop current and voltage to an analog subscriber unit, rather than for serial analog interconnection to another test unit. A direct interconnection would therefore be unlikely to function properly and damage to the test equipment may occur if the analog subscriber lines are directly connected.
The loop circuits of the test equipment devices may be interconnected by way of a transformer to provide DC isolation of the respective analog loop circuits. However, even if the test equipment devices were connected in a serial fashion by way of a transformer, there is no mechanism provided by which the supervisory signaling can be passed from one test unit through an intermediate test unit to a subscriber unit and vice versa. Of particular concern is the conveyance of an on-hook/off-hook indication from an analog subscriber unit. The test equipment directly connected to the analog subscriber unit will be able to detect the loop status (on-hook/off-hook status), however, the loop status cannot be passed to the second, serially connected test equipment. A manual switch configured to be normally open--indicating an on-hook condition--may be included in one or both of the interconnected loop circuits to allow a user to intervene by closing the switch thereby providing an off-hook indication to one or both of the test equipment devices. When the user determines that one unit is initiating a call (such as by detecting an audible ringback signal from the calling subscriber unit), the user may make the necessary connections to complete the circuit. A similar disadvantage is the propagation of a ring signal to the analog subscriber unit. A central office is not designed to receive or propagate a high voltage ring signal.
One disadvantage with this solution is the necessity of manual user intervention. This is a particular disadvantage in testing procedures that are automated by using a computer to remotely control the particular line impairments or other parameters of the test configuration which requires repeated call initiation and termination over a significant time frame. A further disadvantage is that the user may not be able to determine if a call has in fact been initiated. This is particularly difficult if the digital subscriber unit initiates the call because there might not be an audible ringback signal present to indicate call initiation.
It would therefore be desirable to have an improved line performance test system.